2,518 research outputs found
The Origin of Wavelength-Dependent Continuum Delays in AGNs - a New Model
A model of wavelength-dependent lags in optical continuum variability of AGNs
is proposed which avoids the problems of the popular ``lamp-post'' model.
Rather than being due to reprocessing of high-energy radiation from a
hypothetical source above the accretion disk, the wavelength-dependent delays
observed from the B to I bands are instead due to contamination of an
intrinsically coherently variable continuum with the Wien tail of the thermal
emission from the hot dust in the surrounding torus. The new model correctly
gives the size, wavelength dependence, and luminosity dependence of the lags,
and quantitatively predicts observed color hysteresis. The model also explains
how the measured delays vary with epoch of observation. There must also be
contamination by scattered light and this can be detected by a lag in the
polarized flux.Comment: To appear in "The Central Engine of Active Galactic Nuclei", ed. L.
C. Ho and J.-M. Wang (San Francisco: Astronomical Society of the Pacific). 4
pages. 2 diagram
Accurate AGN black hole masses and the scatter in the M_{bh} - L_{bulge} relationship
A new empirical formulae is given for estimating the masses of black holes in
AGNs from the H beta velocity dispersion and the continuum luminosity at 5100
Angstroms. It is calibrated to reverberation-mapping and stellar-dynamical
estimates of black hole masses. The resulting mass estimates are as accurate as
reverberation-mapping and stellar-dynamical estimates. The new mass estimates
show that there is very little scatter in the M_{bh} - L_{bulge} relationship
for high-luminosity galaxies, and that the scatter increases substantially in
lower-mass galaxies.Comment: In press in "Co-Evolution of Central Black Holes and Galaxies", IAU
Symposium Proceedings No. 267, eds. B. M. Peterson, R. S. Somerville, & T.
Storchi-Bergmann. Cambridge University Press, 2010. 1 figur
The origin of the relationship between black hole mass and host galaxy bulge luminosity
There is a strong decrease in scatter in the black hole mass versus bulge
luminosity relationship with increasing luminosity and very little scatter for
the most luminous galaxies. It is shown that this is a natural consequence of
the substantial initial dispersion in the ratio of black hole mass to total
stellar mass and of subsequent galaxy growth through hierarchical mergers.
"Fine-tuning" through feedback between black hole growth and bulge growth is
neither necessary nor desirable.Comment: In press in "The First Stars and Galaxies: Challenges for the Next
Decade", America Institute of Physics Conf. Proc., eds. D. Whalen, V. Bromm,
& N. Yoshida. 1 figure
Line Shifts, Broad-Line Region Inflow, and the Feeding of AGNs
Velocity-resolved reverberation mapping suggests that the broad-line regions
(BLRs) of AGNs can have significant net inflow. We use the STOKES radiative
transfer code to show that electron and Rayleigh scattering off the BLR and
torus naturally explains the blueshifted profiles of high-ionization lines and
the ionization dependence of the blueshifts. This result is insensitive to the
geometry of the scattering region. If correct, this model resolves the
long-standing conflict between the absence of outflow implied by
velocity-resolved reverberation mapping and the need for outflow if the
blueshifting is the result of obscuration. The accretion rate implied by the
inflow is sufficient to power the AGN. We suggest that the BLR is part of the
outer accretion disk and that similar MHD processes are operating. In the
scattering model the blueshifting is proportional to the accretion rate so
high-accretion-rate AGNs will show greater high-ionization line blueshifts as
is observed. Scattering can lead to systematically too high black hole mass
estimates from the C IV line. We note many similarities between narrow-line
region (NLR) and BLR blueshiftings, and suggest that NLR blueshiftings have a
similar explanation. Our model explains the higher blueshifts of broad
absorption line QSOs if they are more highly inclined. Rayleigh scattering from
the BLR and torus could be more important in the UV than electron scattering
for predominantly neutral material around AGNs. The importance of Rayleigh
scattering versus electron scattering can be assessed by comparing line
profiles at different wavelengths arising from the same emission-line region.Comment: 10 pages, 7 figures, Astrophysical Journal in press. The only changes
from the previous version are to include some additional discussion of the
plausibility of supersonic inflow velocities (see section 5.2) and some
additional reference
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